1. Field of the Invention
The invention relates to an X-ray examination/diagnostic apparatus including an X-ray source and an X-ray image detection device which are mutually displacable with respect to an object to be examined, and more particularly, to a method and apparatus for processing and displaying an image of the object, wherein the object is larger than the imaging field of the X-ray image detection device. The invention finds a preferred field of application in angiography, in particular for visualizing at least a large proportion of a patient's blood vessels and circulatory system.
2. Description of the Prior Art
In angiography, it is extremely useful to be able to display as a continuous image a large part of the arterial and/or venous system such as, in particular, the entire lower arterial and venous system. In the past, examinations of this type have been performed by means of an installation comprising an X-ray source placed at a considerable height above the patient in order to irradiate the entire region to be visualized as well as a film transfer mechanism for taking a series of adjacent photographs corresponding to different portions of this region. This system suffers from the double disadvantage of high operating costs arising from the considerable lengths of photographic film used and also arising from extensive irradiation of the patient. Furthermore, it is not always easy to read the document by reason of the variations in quality of the successive images. Additionally, such large films are difficult to handle. More recently, with the appearance of image intensifiers which tend to replace films, it has been proposed to take a number of digital acquisitions in respect of different relative positions between the source-detector assembly and the patient-supporting table and to reproduce these digital images in a non-continuous manner on a single photographic document of distinctly smaller size than the series of photographs described above for an equivalent resolution of the image. Furthermore, the X-radiation dose applied to the patient is reduced to an appreciable extent. However, this type of presentation calls for an effort of mental reconstruction of the image on the part of the physician, which is considered as a restrictive condition.
Presently, digital X-ray examination apparatus for vascular imaging are now widely used and well known in the literature, e.g., from U.S. Pat. No. 4,204,225 entitled REAL TIME DIGITAL X-RAY SUBTRACTION IMAGING. In such a device for digital subtraction imaging, X-ray images of a portion of a patient are recorded both before a contrast medium is injected into the patient as well as after injection of the contrast medium into the patient. Both sets of images are stored and digitized and then processed so that the images without the contrast medium (referred to hereinafter as mask images) are subtracted from the contrast-filled images (referred to hereinafter as contrast images) with a result that the contrast-filled vascular system becomes highly visible upon display of the processed images. During examination of, for example, parts of the body of the patient which do not fit within a single image field of the X-ray image detection device, the images are acquired from a plurality of different locations along the length of the patient. Programmable stepping tables or gantrys provide the required relative movement between the X-ray examination device and the patient.
It is an object of the invention to facilitate X-ray examinations and diagnosis for larger parts of the body larger than the optical input of the X-ray image detection device, by appropriate processing and display of a series of X-ray images acquired along the length of the body.